1. Field of the Invention
This invention relates to a draw furnace; and more particularly, a fiber optic draw furnace for heat treating a glass preform or drawing optical fibers from a preform.
2. Background of the Invention
When thermally treating a high purity optical preform for drawing into optical fiber, inflow of oxygen and other atmospheric gases into the furnace can cause substantial production problems. Oxygen causes accelerated wear of graphite furnace parts and the generation of loose silicon carbide and graphite particulates that contaminate and degrade the physical strength of a finished fiber. The interior of a furnace is traditionally prone to contamination by ambient particulate matter. Common particulates have been demonstrated to reduce the optical and physical quality of the finished fiber. Oxygen from the atmosphere or silicon dioxide (Sio.sub.2) from the preform can react with materials used to line the furnaces, commonly at operating temperatures, shortening the useful life of the furnace.
In an attempt to overcome problems, prior art furnaces use two chambers separated by a partition such as a sliding door or shutter door. The door can be opened to provide access from a first chamber into a second thermal treating chamber once a preform is first decontaminated and temperature controlled in an environmentally controlled chamber. The atmosphere in both chambers is commonly purged of oxygen and particulate matter by using an inert gas.
However, this sequence of opening and closing two chambers has only been known to be actuated manually, while also inherently entailing the time-consuming steps of purging, heating, and cooling the chambers. Heating and cooling results from shutting down the heater, in the control chamber of the furnace. Shutting down the heater is often necessary to avoid higher temperature reactions at the control chamber stage when atmospheric gases and particulate matter are encountered and purged. Clearly, shutting down the furnace results in lost fiber production time as a preform cannot be immediately inserted into the furnace.
Typically, prior art furnaces require about a half-hour to cool, and an additional half-hour to reheat to furnace operating temperature. This represents valuable production time during which fibers are not being produced.
The reader is referred to U.S. Pat. No. 5,032,079, German patent DE 3,925,961, Japanese patent JP 6-199536, and Great Britain patent 1,523,595 for examples of other known prior art optical furnaces.